BBC News has the story at Hamburger icon: How these three lines mystify most people - BBC News.
The Wikipedia writes: "The triple bar, ≡, is a symbol with multiple, context-dependent meanings...."
The triple bar is hardly a modern-day invention, being already predated by two other significant "prior art" uses in long-gone eras, as noted at the Wikipedia:
- ☰, Qián, the trigram of the I Ching that consists of three unbroken lines
- Ξ, capital letter Xi of the Greek alphabet
The capital letter Xi Ξ of the Greek alphabet, as we analyzed in our book, The Syllabic Origins of Writing and the Alphabet, has comparable signs in Linear B, Old Elamite, the Cypriot Syllabary, and the Samekh of the Hebrew and Phoenician alphabets, and evolved as a symbol "out of fish signs and signs representing fish drying racks having two or three levels upon which to place the fish" as also found in symbols in Sumerian, Pharaonic Egyptian, and Luwian.
Indeed, the Hebrew Samekh means "support", i.e. the function of the fish rack, a concept which surely is related to the I Ching idea of a triple bar as meaning "heaven, sky", as the "supported" firmament of the ancients.